Generally, a plasma display panel (hereinafter, referred to as a PDP) incorporates many features in that it can be made thin, it can comparatively easily display a large screen, it can provide a wide-range viewing angle, it can provide a high response speed or a like. Therefore, in recent years, it is used for a wall-mounted television, public display plate or a like in a form of a flat display device. The PDP can be classified, by operation mode, into two groups; one being a DC (direct current)-type PDP adapted to be operated with its electrode being exposed to discharge space (that is, to discharge gas) and in a direct current discharging condition and another being an AC-type PDP adapted to be operated with its electrode coated with dielectric layers and without its electrode being directly exposed to discharging gas and in an alternating current discharge condition. In the DC-type PDP, discharge occurs while a voltage is being applied. In the AC-type PDP, discharge is sustained by changing a polarity of a voltage to be applied. Moreover, in the AC-type PDP, a number of electrodes contained in one cell is two or three.
Configurations and driving method of a conventional three-electrode AC-type PDP are described below. FIG. 2 is a cross-sectional view of one example of a cell used for a conventional PDP. The conventional three-electrode AC-type PDP includes a front substrate 20 and a rear substrate 21 both of which are placed opposite to each other, a plurality of X electrodes 22, Y electrodes 23, and data electrodes 29 being disposed between the front substrate 20 and the rear substrate 21, and display cells being disposed at each of intersections of the X electrodes 22, the Y electrodes 23, and the data electrodes 29.
As the front substrate 20, a glass substrate or a like is used. Each of the X electrodes 22 and each of the Y electrodes 23 are placed at a specified interval. On these X electrodes 22 and Y electrodes 23 is formed a transparent dielectric layer 24. On the transparent dielectric layer 24 is formed a protective layer 25 made up MgO (Magnesium oxide) or a like adapted to protect the transparent dielectric layer 24 from discharging. On the other hand, as the rear substrate 21, a glass substrate or a like is used. Each of the data electrodes 29 is so mounted as to be perpendicular to each of the X electrodes 22 and to each of the Y electrodes 23.
On the data electrodes 29 is formed a white dielectric layer 28. On the while dielectric layer 28 is formed a phosphor layer 27. Between front substrate 20 and rear substrate 21 is placed a partition wall (not shown) at a specified interval in parallel to a face of paper. The partition wall is used to secure discharge space 26 and to demarcate pixels. The discharge space 26 is filled, in a sealed manner, with mixed gas such as He (Helium), Ne (Neon), Xe (Xenon) or a like, as discharge gas to be used for discharge. The conventional three-electrode AC-type PDP having such configurations as described above is disclosed in SID (Society for Information Display) 98 DIGEST (P279–281, May, 1998).
FIG. 3 is a plan view of the conventional three-electrode AC-type PDP. As shown in FIG. 3, at each of intersections of each electrode Xi (i=1 to m) making up the X electrodes 22 and each electrode Yi (i=1 to m) making up the Y electrodes 23 and each electrode Dj (j=1 to n) making up the data electrode 29, each of display cells is disposed. These display cells are placed in a matrix form.
Next, a conventional method for driving a PDP will be described below. As the method for driving the PDP, a scanning/sustaining separation method (ADS method) an which a scanning period and a sustaining period are separated is in the present mainstream. However, this method requires a plurality of sub-fields (SF) for displaying a gray shade and also requires the scanning period for each of the SFs. Therefore, if the number of gray scales Or the number of scanning lines is increased, the scanning period forms an increasing proportion of one field and, as a result, the sustaining period forms a decreasing proportion of one field, causing low luminance in display. To solve this problem, an alternative method for driving the PDP by which the gray shade can be displayed by one time scanning without using such SFs is proposed. The method of this type for driving the PDP is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 9-81073. The scanning/sustaining separation method will be described below. FIG. 1 is a diagram showing waveforms explaining driving operations of the conventional three-electrode AC-type PDP. One field 1 is made up of three periods including a preliminary discharge period 2, a scanning period 3, and a sustaining period 4.
First, the preliminary discharge period 2 will be described. A preliminary discharge pulse 5 with positive polarity is applied to the X electrode 22 and a preliminary discharge pulse 6 with negative polarity is applied to the Y electrode 23. This enables resetting of irregularity caused by light emitting conditions in a pre-field period, in a state in which wall charges occur at a final stage of a pre-SF and enables all pixels to be forcedly discharged, thus providing a priming effect which induces subsequent writing discharge to occur at a lower voltage. Moreover, though, in the example shown in FIG. 1, both the preliminary discharge pulses 5 and 6 are applied once with same timing, in some cases, two kinds of pulses each having a different role are applied, that is, a priming pulse to cause all pixels to be discharged and priming effects to be implemented is applied after a sustainment extinguishing pulse to cause the stat of the pre-field to be reset has been applied. At this point, in some cases, a different sustainment extinguishing pulse is applied not only once but also two or more numbers of times. Furthermore, through, in the example shown in FIG. 1, to extinguish the wall charge produced by the preliminary discharge, a self-extinguishing process by using a fall of each of the preliminary discharge pulses is employed, in some cases, a preliminary discharge extinguishing pulse is applied to extinguish these wall charges separately. In some cases, the preliminary discharge extinguishing pulse is also applied not only once but also two or more numbers of times. Moreover, in some cases, these pulses are applied to other electrodes. In any case, the wall charge on the dielectric layer produced by the preliminary discharge is extinguished or is controlled to be propre in quantity.
Next, the scanning period 3 is described below. During the scanning period 3, a scanning pulse 8 with negative polarity is applied sequentially to each of electrodes (X1 to Xm) making up the X electrodes 22. At the same time when the scanning pulse 8 is applied, a data pulse 10 is applied, so as to correspond to a display pattern, to each of electrodes (D1 to Dn) making up the data electrodes 29. The data pulse 10 changes a pulse voltage depending on gray scale to be displayed. In the case of a gray scale with low luminance, the pulse voltage is set to a low level and, then, the voltage is boosted as luminance becomes higher. When application of the scanning pulse 8 is completed, a wall charge being almost equivalent to a potential difference between the scanning pulse 8 and the data pulse 10 is accumulated by writing discharge. Therefore, a large amount of the wall charge is accumulated in a pixel into which a signal with high luminance has been input and a small amount of the wall charge is accumulated in the pixel into which a signal with low luminance has been input. A scanning base voltage 7 being applied to the scanning electrode 22 during the scanning period is applied to prevent erroneous discharging that may occur, after the writing discharge, between the X electrode 22 and the Y electrode 23 of a pixel being adjacent to the X electrode 22 (that is, between non-discharging gaps).
After the scanning pulse 8 has been applied to all lines, a sustaining period 4 starts. Each of the sustaining pulses 11 is applied alternately to all of the X electrodes and all of the Y electrodes. Voltages of the sustaining pulses 11 are increased step by step during the sustaining period. As a result, potential difference between the X electrode 22 and the Y electrode 23 increases as their polarities are reversed. However, this voltage is set to a level at which discharge does not occur. Therefore, since an amount of the wall charge is small in a pixel in which writing discharge has not occurred, even when the sustaining pulses are applied, no discharge occurs. On the other hand, in the pixel in which the writing discharge occurs, the wall charge is accumulated in the X electrode 22 to correspond to a gray shade. During the sustaining period 4, a voltage resulting from superposition of a voltage produc d by the wall charge accumulated in th X electrode 22 by the writing discharge on the potential difference between the sustaining pulses 11 is applied between the X electrode 22 and the Y electrode 23. Since the sustaining voltage is increased step by step, when it exceeds a start voltage for surface discharge at some point in time, the surface discharge occurs between the X electrode 22 and the Y electrode 23. At this time, since a data bias voltage 12 is applied to the data electrode 29, no opposite discharge occurs. Once the surface discharge occurs, a large amount of the wall charge with reverse polarity is accumulated in the X electrode 22 and the Y electrode 23. The accumulated wall charge, since the subsequent sustaining pulse voltage with reverse polarity is superposed on the wall charge, produces a large potential difference, thus causing the surface discharge with reverse polarity to occur again and a large amount of the wall charge with reverse polarity to be again accumulated. Thus, once the surface discharge occurs, every time the polarity of the sustaining pulse is reversed, the surface discharge is repeated until the sustaining period 4 ends.
A timing of a start of the surface discharge changes depending on an amount of the wall charge accumulated by the writing discharge. That is, if the amount of the wall charge is small, the sustaining pulse with a high voltage is required and the surface discharge does not start until the sustaining pulse 11 with the high voltage produced at a later stage of the sustaining period 4 is applied, while, if the amount of the wall charge is large, the surface discharge starts when the sustaining pulse with a low voltage is applied. Thus, a period while light is emitted (that is, the period while discharge occurs) can be changed during the sustaining period 4 depending on the amount of the wall charge. The amount of the wall charge is produced by the writing discharge at a time of writing depending on the gray scale to be displayed. Thus, a period while light is emitted can be controlled depending on the gray scale. The gray scale is displayed under such control.
As described above, in the conventional example, the writing discharge occurs only in the X electrodes 22 during the scanning period, and lighting or non-lighting is decided depending on a difference between the amount of the wall charge produced in the X electrode 22 and the amount of the wall charge produced in the Y electrode 23. In order to realize this, it is required to set the sustaining pulse voltage in a prescribed range.
For lighting pixels, the sustaining pulse voltage is required to be the minimum sustaining voltage Vsm or higher at which the sustaining discharge continues. On the other hand, for non-lighting pixels, since the discharge is required not to occur while the wall charge is not produced, the sustaining pulse voltage is required to be lower than a discharge start voltage Vf. Although depending on configurations and dimensions of a cell, materials of gas, etc., generally the minimum sustaining voltage Vsm is around 130 V and the discharge start voltage Vf is around 190 V. Therefore, the sustaining pulse voltage is allowed in the range of around 130 to 190 V. As in the conventional example described above, when the display of the gray scale is performed by scanning of one time, the sustaining pulse voltage is required to be set in some stages. Then, the upper limit and the lower limit are given and, as a result, the number of gray scales to be set is only a few in the range of 60 V.
Moreover, for example, when four gray scales are displayed, the sustaining pulse voltages are set in the range of 140 V to 180 V at a step of 20 V. A 0th gray scale gray scale is defined as black and a third gray scale is defined as while. Then, if it is assumed that the sustaining discharge occurs at a time when the sustaining pulse voltage in a first gray scale reaches 180 V, a potential difference of 20 V as the sustaining pulse voltage is too small a margin, and accordingly when a sustaining pulse voltage of 160 V is applied, weak discharge may occur. Therefore, conditions of the wall charge at a time of writing (the amount of the wall charge decreases) and, in some cases, the sustaining discharge may occur even when a sustaining pulse 11 of 180 V is applied. This fact causes flicker in display. Furthermore, since luminance in the sustaining discharge is dependent on the sustaining pulse voltage, the luminance level at 140 V is low and is likely to become unstable. Moreover, not only the luminance level is simply proportional to the number of pulses, but also the total luminance level is suppressed so as to be low.
In view of the above, it is an object of the present invention to provide a method for driving an AC plasma display panel which makes it possible to reduce flicker in display and have stable and high luminance.